(Opto)electronic arrangements are being used ever more frequently in commercial products. Arrangements of this kind comprise inorganic or organic electronic structures, for example organic, organometallic or polymeric semiconductors or else combinations thereof. These arrangements and products are rigid or flexible according to the desired use, there being an increasing demand for flexible arrangements. Arrangements of this kind are produced, for example, by printing methods such as relief printing, gravure printing, screen printing, flat printing, or else “non-impact printing”, for instance thermal transfer printing, inkjet printing or digital printing. In many cases, however, vacuum methods, for example chemical vapour deposition (CVD), physical vapour deposition (PVD), plasma-enhanced chemical or physical deposition (PECVD) methods, sputtering, (plasma) etching or vaporization, are used, in which case the structuring is generally effected by means of masks.
Examples of (opto)electronic applications that have already been commercialized or are of interest in terms of their market potential include electrophoretic or electrochromic assemblies or displays, organic or polymeric light-emitting diodes (OLEDs or PLEDs) in readout and display devices or as lighting, electroluminescent lamps, light-emitting electrochemical cells (LEECs), organic solar cells, preferably dye or polymer solar cells, inorganic solar cells, preferably thin-film solar cells, especially based on silicon, germanium, copper, indium and selenium, perovskite solar cells, organic field-effect transistors, organic switching elements, organic optical amplifiers, organic laser diodes, organic or inorganic sensors or else organic- or inorganic-based RFID transponders.
Further applications of encapsulating adhesive tapes are known in the field of battery technology, especially in the field of flexible microbatteries and thin-film batteries, very particularly those comprising lithium-containing cathodes, anodes or electrolytes.
Accordingly, in this document, an organic (opto)electronic arrangement is understood to mean an electronic arrangement which comprises at least one electronically functional, at least partly organic constituent—for example organometallic compounds—or wherein the electronically functional structure has a thickness of less than 20 μm.
A technical challenge for the achievement of adequate lifetime and functioning of (opto)electronic arrangements in the field of inorganic and/or organic (opto)electronics, but very particularly in the field of organic (opto)electronics, is considered to be protection of the components present therein from permeates. Permeates may be a multitude of low molecular weight organic or inorganic compounds, especially water vapour and oxygen.
A multitude of (opto)electronic arrangements in the field of inorganic and/or organic (opto)electronics, very particularly in the case of use of organic raw materials, are sensitive both to water vapour and to oxygen, the penetration of water or water vapour being classified as a major problem for many arrangements. During the lifetime of the electronic arrangement, therefore, protection by encapsulation is required, since the performance otherwise declines over the period of use. For example, oxidation of the constituents can result, for instance, in a severe reduction in luminance in the case of light-emitting arrangements such as electroluminescent lamps (EL lamps) or organic light-emitting diodes (OLEDs), in contrast in the case of electrophoretic displays (EP displays), or in efficiency within a very short time in the case of solar cells.
In order to achieve very good sealing, specific barrier adhesives are used (also referred to as adhesives having water vapour barrier properties). A good adhesive for the sealing of (opto)electronic components has low permeability to oxygen and especially to water vapour, has sufficient adhesion on the arrangement and can adapt well thereto.
The barrier action is typically characterized by reporting the oxygen transmission rate (OTR) and the water vapour transmission rate (WVTR). The respective rate indicates the area- and time-based flow of oxygen or water vapour through a film under specific conditions of temperature and partial pressure and possibly further measurement conditions such as relative air humidity. The smaller these values, the better the suitability of the respective material for encapsulation. The reported permeation is not based solely on the values of WVTR or OTR but always also includes specification of the minimum path length of the permeation, for example the thickness of the material, or normalization to a particular path length.
The permeability P is a measure of the ability of gases and/or liquids to permeate through a body. A low P value indicates a good barrier action. The permeability P is a specific value for a defined material and a defined permeate under steady-state conditions with a particular permeation path length, partial pressure and temperature. The permeability P is the product of the diffusion term D and solubility term S: P=D*S.
The solubility term S predominantly describes the affinity of the barrier adhesive for the permeate. In the case of steam, for example, a small value of S is achieved by hydrophobic materials. The diffusion term D is a measure of the mobility of the permeate in the barrier material and is directly dependent on properties such as molecular mobility or the free volume. It is often the case that relatively low values are achieved for D in highly crosslinked or highly crystalline materials. However, highly crystalline materials are generally not very transparent, and greater crosslinking leads to lower flexibility. The permeability P typically rises with an increase in molecular mobility, for instance when the temperature is increased or the glass transition point is exceeded.
Attempts to increase the barrier action of an adhesive have to take account of both parameters D and S, especially with regard to the effect on the permeability of water vapour and oxygen. In addition to these chemical properties, effects of physical influences on permeability also have to be considered, especially the mean permeation path length and interfacial properties (adaptation characteristics of the adhesive, adhesion). The ideal barrier adhesive has low D values and S values combined with very good adhesion on the substrate.
A low solubility term S alone is usually insufficient to achieve good barrier properties. A particular classic example of this is that of siloxane elastomers. The materials are extremely hydrophobic (small solubility term), but by virtue of the free rotation about the Si—O bond (large diffusion term) have a comparatively small barrier action against water vapour and oxygen. For good barrier action, a good balance is thus needed between the solubility term S and diffusion term D.
There have been descriptions of barrier adhesives based on styrene block copolymers and resins having maximum hydrogenation levels (see DE 10 2008 047 964 A1). Permeation values (WVTR) of commonly used adhesive systems are also reported here (measured at 37.5° C. and 90% relative humidity). Typical acrylate-based pressure-sensitive adhesives are in the range between 100 g/m2 d and 1000 g/m2 d. Because of the high mobility of the chains, pressure-sensitive silicone adhesives have even higher permeation values for water of more than 1000 g/m2 d. If styrene block copolymers are used as elastomer component, WVTR values in the range from 50-100 g/m2 d are achieved for unhydrogenated or incompletely hydrogenated systems and values below 50 g/m2 d for hydrogenated systems (for example SEBS). Particularly low WVTR values of less than 15 g/m2 d are achieved both with pure poly(isobutylene) elastomers or block copolymers of styrene and isobutylene.
One means of improving the barrier action again is to use substances that react with water or oxygen. Oxygen or water vapour that penetrate into the (opto)electronic arrangement are then bound chemically or physically, preferably chemically, by these substances and hence increase the breakthrough time (“lag time”). The substances are referred to in the literature as “getters”, “scavengers”, “desiccants” or “absorbers”. Only the term “getters” is used hereinafter. One way in which the penetrating water is bound is by physical means via adsorption typically on silica, molecular sieves, zeolites or sodium sulphate. Water is bound chemically via alkoxysilanes, oxazolidines, isocyanates, barium oxide, phosphorus pentoxide, alkali metal and alkaline earth metal oxides (for example calcium oxide), metallic calcium or metal hydrides (WO 2004/009720 A2). However, some fillers are unsuitable for transparent bonding, for example of displays, since the transparency of the adhesive is reduced.
Such getters that have been described in adhesives are mainly inorganic fillers, for example calcium chloride or various oxides (cf. U.S. Pat. No. 5,304,419 A, EP 2 380 930 A1 or U.S. Pat. No. 6,936,131 A). Adhesives of this kind are dominant in edge encapsulation, i.e. in cases where only edges have to be bonded. However, adhesives comprising such getters are unsuitable for full-area encapsulation, since, as detailed above, they reduce transparency.
Organic getters have also been described in adhesives. For example in EP 2 597 697 A1, in which polymeric alkoxysilanes are used as getters. Numerous different silanes as getters in adhesives are mentioned in WO 2014/001005 A1. According to this document, the maximum amount of getter to be used is 2% by weight, since the sensitive electronic assembly to be encapsulated would be damaged in the case of use of higher amounts of getter. A problem is that the organic getter materials used are usually very reactive and lead to damage (called “dark spots”) on contact with the sensitive organic electronics in the full-area encapsulation. Adhesives comprising such getters are thus suitable only for edge encapsulation, where impairment of transparency is unimportant.
In summary, getter materials are, for example, salts such as cobalt chloride, calcium chloride, calcium bromide, lithium chloride, lithium bromide, magnesium chloride, barium perchlorate, magnesium perchlorate, zinc chloride, zinc bromide, silicas (for example silica gel), aluminium sulphate, calcium sulphate, copper sulphate, barium sulphate, magnesium sulphate, lithium sulphate, sodium sulphate, cobalt sulphate, titanium sulphate, sodium dithionite, sodium carbonate, potassium disulphite, potassium carbonate, magnesium carbonate, titanium dioxide, kieselguhr, zeolites, sheet silicates such as montmorillonite and bentonite, metal oxides such as barium oxide, calcium oxide, iron oxide, magnesium oxide, sodium oxide, potassium oxide, strontium oxide, aluminium oxide (activated alumina), and also carbon nanotubes, activated carbon, phosphorus pentoxide and silanes; readily oxidizable metals, for example iron, calcium, sodium and magnesium; metal hydrides, for example calcium hydride, barium hydride, strontium hydride, sodium hydride and lithium aluminium hydride; hydroxides such as potassium hydroxide and sodium hydroxide, metal complexes, for example aluminium acetylacetonate; and additionally organic absorbers, for example polyolefin copolymers, polyamide copolymers, PET copolyesters, anhydrides of mono- and polycarboxylic acids such as acetic anhydride, propionic anhydride, butyric anhydride or methyltetrahydrophthalic anhydride, isocyanates or further absorbers based on hybrid polymers, which are usually used in combination with catalysts, for example cobalt, further organic absorbers, for instance lightly crosslinked polyacrylic acid, polyvinyl alcohol, ascorbates, glucose, gallic acid or unsaturated fats and oils.
In accordance with their function, the getter materials are preferably used as essentially permeate-free materials, for example in water-free form. This distinguishes getter materials from similar materials which are used as filler. For example, silica is frequently used as filler in the form of fumed silica. If this filler, however, is stored as usual under ambient conditions, it absorbs water even from the environment and is no longer able to function as a getter material to an industrially utilizable degree. It is only silica that has been dried or kept dry that can be utilized as getter material. However, it is also possible to use materials partly complexed with permeates, for example CaSO4*½H2O (calcium sulphate hemihydrate) or partly hydrated silicas which exist by definition as compounds of the general formula (SiO2)m*nH2O.
As described above, silicas are understood to mean compounds of the general formula (SiO2)m*nH2O. This is silicon dioxide produced by wet-chemical, thermal or pyrogenic methods. More particularly, suitable getter materials among the silicas are silica gels, for example silica gels impregnated with cobalt compounds as moisture indicator (blue gel), and fumed silicas.
In the case of full-area encapsulation, there are opposing requirements of high reactivity of the getter in order to ensure maximum protection from the penetration of water vapour on the one hand and a low reactivity of the getter in order not to damage the sensitive organic electronics on the other hand.
It was therefore an object of the invention to provide an adhesive which has a long breakthrough time (>1100 h (storage at 60° C./90% r.h.) and >220 h (storage at 85° C./85% r.h.)) and which can be used over the full area for encapsulation of assemblies from organic electronics, without damaging the sensitive organic electronics.